Ionizing radiation has been used in the oilfield for well over half a century to enable measurements not possible with other formation probing techniques (such as electrical resistivity, acoustic, electro-magnetic, NMR, etc.). The broad types or kinds of ionizing radiation include charged particles, X-rays, gamma-rays, and neutrons.
Historically, the first method employed to produce ionizing radiation for oilfield applications involved radioactive sources. These radioactive sources (e.g. 137Cs, 241Am, 252Cf, 238Pu, etc.) were chosen, based on the application, for their emitted radiation type and in quantities sufficient for the desired intensity. Few naturally occurring radioactive sources emit neutrons during their decay, generally as a consequence of spontaneous fission. One example of a radioactive material with a significant probability for spontaneous fission is 252Cf. While 252Cf is used in some applications, its short half-life of only 2.6 y is generally considered a drawback. Many neutron sources rely on the generation of neutrons through a nuclear reaction, typically the 9Be(alpha, n)12C reaction. Such sources consist of an alpha emitter, such as 241Am or 238Pu, mixed with Be-powder. The small nuclear reaction probability necessitates the use of a large amount of radioactive alpha emitter to generate a sufficient neutron output.
Most pulsed neutron generators generate neutron radiation through the reaction of energetic deuterium with tritium (d-T) reaction. However, other reactions such as d-D and t-T can be used as well.
To overcome disadvantages involved in using radioactive sources, and to expand the range of possible measurement techniques, electronic radiation generators have been developed over the last several decades. Electronic radiation generators offer numerous benefits, including safety and high yield, over chemical naturally radioactive sources. The radiation generated is directly coupled to the energy imparted to the charged particles employed in the radiation producing reactions. As a result, electrostatic electronic radiation generators employ high voltage power supplies and high voltage insulation.
To fit the borehole geometry of the oilfield environment, electronic radiation generators are typically of a substantial length on the order of a few feet. This minimum length can become a disadvantage, constraining possible tool configurations, both in terms of fit as well as formation measurement capabilities.
Electronic generators function by accelerating and colliding charge particles in sealed “vacuum” tubes to produce the required neutrons or x-rays. At present, most of the accelerators used to produce radiation in the oilfield use direct current (DC) electric fields, operating by creating a sufficiently large voltage (potential) difference between a source of charged particles and a target. Once the source is turned ON, charged particles entering the region of high voltage (HV) are accelerated to the desired energy and caused to collide with a target to produce radiation. When either the source and/or the voltage is/are turned OFF, the radiation stops. The charged particles to be accelerated can be electrons (to produce X-rays), protons and other ions (to produce gamma rays, neutrons and other charged particles).
X-rays are generated by directing energetic electrons onto a target. The scattering of the energetic electrons on the target leads to the emission of Bremsstrahlung x-rays and characteristic x-rays of the target material.
Gamma-rays can be created through a nuclear reaction of an energetic charged particle with a suitable target material. One example is the reaction 19F(p,αγ)16O. his reaction produces high energy gamma rays (e.g. 6.13 MeV) and already takes place at proton energies below 400 keV.
The intensity and energy of the radiation produced depends on the high voltage used to accelerate the charged particles. Typically, the higher the “high voltage”, the higher the intensity and/or energy of the resulting radiation. This requires use of high voltage power supplies (HVPS) capable of producing from tens to hundreds of kilovolts. The most common power supply design employed is based on the Cockcroft-Walton voltage multiplier, also known as the Greinacher multiplier. It includes an array of stages in series, shaped like a ladder, with each stage comprising a pair of capacitors and a pair of diodes (see FIG. 1).
During each ON cycle, an A/C current supplied by step up transformer 100 charges each respective capacitor 102, according to the rectification provided by the diodes 104. The maximum admissible voltage for each stage depends on the capacitor and diode component ratings as well as on the AC voltage provided by the secondary of the transformer 100. To achieve a given high voltage, a ladder is constructed such that, neglecting losses, etc., enough stages are used to sum up to the desired voltage.
One electrical design constraint for high voltage power supplies is that electrical hold off between the high voltage and ground must be sufficient over the whole length of the power supply. Given the limited diameter available in downhole tools, the stage component size must be small enough to accommodate the necessary surrounding high voltage insulation over a limited radial distance. Typically, the size of high voltage components increases with their high voltage rating. A tradeoff is made between the stage (and component) size and the necessary high voltage insulation based upon the absolute voltage to neighboring ground potential. For borehole-size radiation generators, high voltage capacitor ratings are typically around a few (e.g., five) kilovolts. Typically multi-layer capacitors are used to achieve the required capacitance, and are generally rectangular or box-shaped with radial leads. Thus, to produce from tens to hundreds of kilovolt requires a power supply with several dozens of stages. When the stages are placed end to end, the “ladder” configuration adds up to a significant overall length for the power supply.
Currently, the most frequently employed electronic radiation generator in the oilfield is the pulsed neutron generator (PNG). Aside from the supporting low voltage electronics (current and pulsing supplies), a PNG comprises a high voltage power supply 202 consisting of a step-up transformer 100 and a high voltage multiplier 216, a sealed neutron generator tube 204 and a high voltage insulation system 206. The sealed neutron generator tube 204 includes an ion source 208 which is at or near ground potential, and separated from a target 210 by an HV acceleration column. The configuration of PNG is generally cylindrical, as dictated by the borehole geometry, with the target 210 of the neutron generator tube 204 in contact with the high voltage end of the high voltage power supply 202. The necessary high voltage insulation (solid, liquid and/or gaseous or any combination thereof) surrounds the neutron generator tube 204 and high voltage power supply 202, with the entirety being contained in a hermetically-sealed pressure housing 212. Preferably, the pressure housing is cylindrical in shape. The conventional arrangement of these components is in a rectilinear configuration, where the high voltage power supply is connected in line (on one axis or close to on one axis) to the high voltage end of the neutron generator tube 204 (see FIG. 2.) The typical length of such a generator is often in the range from about 36 to 48 inches, or more.
The considerable length of the PNG can impose significant limitations on the tool design and consequently the feasibility of various measurements. Specifically, tools with limited length may not be able to make use of an electronic radiation source. Such tools include those that use pads (e.g., pad tools to measure formation density or tools in which the generator should be placed in the stabilizer blade of an LWD tool). The measurement limitations pertain to the placement of specialized radiation detectors used to measure the formation response. Many classic nuclear logging tool configurations require placing radiation shielding material (tungsten, boron and many others) as close as possible to the radiation source (i.e., PNG) with a specialized (near) detector adjacent to the shield. The detector configuration (i.e., type, size, shielding, position, etc.) is chosen based on the kind of formation measurement desired. Because of the length of the PNG including the high voltage power supply, only the neutron tube end affords close proximity to the radiation source (tube target). For most practical purposes, this limits the detector placement to a single axial direction relative to the target. If a tool is intended to perform multiple different measurements with the radiation provided by the electronic source, it is likely that not all the required detectors can be placed at the optimal axial and/or radial position. A short radiation source would allow detector placements axially on both sides of the radiation source, and thus a better independent optimization of multiple measurements through optimal placement of multiple detectors. A different approach to achieving a similar result is proposed in U.S. Pat. No. 7,148,471.